Domain size change of spinodal phase separation structure in the sol-gel derived TiO 2 thin film
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TiO2 thin films were prepared with a spinodal phase separation structure ranging from hundreds of nanometers to micrometers in size by the sol-gel dip-coating method from a titanium tetraisopropoxide solution containing polyoxyethylene (20) nonylphenyl ether (NPE-20). It was possible to change the titania domain size of the spinodal phase separation structure by selecting alcohol as the solvent because the polycondensation and the phase separation are severely influenced by its evaporation rate from the surface of the coated film. It was also observed that the titania domains became larger in size as the film thickness increased in the bottom area of the substrate when the dip-coating method was applied, while uniformly sized titania domains were possibly formed by a spin-coating method. Furthermore, the contrast between the TiO2 phase and the air phase was not sharp when the titania domain size was smaller than several hundreds of nanometers. It was possible to prepare TiO2-air interconnective films with the spinodal phase separation structure 200–300 nm in domain size by subsequent HF etching. In addition, combining spinodal phase separation structure (SPSS) TiO2 thin film with different skeleton size has the potential to create TiO2 thin film with high photo-electron conversion efficiency due to its particular structure, higher surface area, and lack of bottleneck for electron transfer.
I. INTRODUCTION
Since it is relatively abundant as a natural resource and a semiconductor with an excellent chemical stability,1,2 TiO2 has been extensively studied as an electrode material for photoelectrochemical cells3–5 and for water decomposition.6 Furthermore, TiO2 can be used for many types of devices such as electrochromic display7,8 and gas sensors.9,10 We have recently reported the preparation of TiO2 thin films with a micrometer-size spinodal phase separation structure (SPSS) by the sol-gel dipcoating method using a surfactant of polyoxyethylene (20) nonylphenyl ether (NPE-20) added to a precursor ethanolic solution. It was considered that NPE-20 is effectively adsorbed on titania oligomers to render them hydrophobic, which would be the driving force of the spinodal phase separation.11 It is known that when a mixture reaches the critical composition of the system, the spinodal phase separation interconnective domain structure develops and remains unbroken for a substantial period of time, and the domains grow in size during the coarsening stage.12 Hence, it is deduced that freezing the morphology
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0027 270
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 1, Jan 2006 Downloaded: 18 Mar 2015
during the early stage of the spinodal phase separation results in the formation of an SPSS with a smaller domain size. In the previous system, ethanol was used as the solvent.11 Therefore, it is possible to form a finer SPSS if the polycondensation reaction and the accompanying phase separation process slowly
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